The image contrast of a computed tomography (CT) image is closely related to spectrum distribution of the X-ray source used in the scan. The conventional CT uses a light source having spectrum distribution for imaging, sometimes, information blur might occur causing two different materials to be presented completely the same on the CT image; in contrast, the dual-energy CT uses two spectrums having different distributions to image objects, thus eliminating information blur caused by single energy spectrum. The dual-energy computed tomography (CT) technology takes advantage of the difference in attenuation of substance in different energies to obtain distribution information about multiple physical characteristics parameters of objects, for example, electron density distribution, equivalent atomic number distribution, and single energy attenuation image in multiple energies. Hence, the dual-energy CT can be used for correction of beam hardening of the conventional CT, obtaining clinical high-contrast spectrum images, detection of special and dangerous objects in industry and security check, etc. Compared to the conventional CT imaging technology, the dual-energy CT is significant to such application fields as medical diagnosis technologies, nondestructive testing and security check because of the breakthrough in imaging function it has made, so it has gained more and more widespread attention in recent years.
In addition, compared to the convention single energy CT technology, the dual-energy CT imaging technology can not only provide the attenuation coefficient and geometrical structure information of the object to be inspected, but also provide the material composition information thereof. Therefore, in the field of security check, the dual-energy CT technology can provide the electron density and effective atomic number information of the object to be inspected, thereby identifying dangerous substances. Moreover, in the field of medical treatment, the dual-energy CT can provide images of different tissue components, such as bone, soft tissue, contrast agent, etc.
The dual-energy CT system at present can mainly be implemented in three ways, i.e. dual-source dual-detector, single source double layer detector, fast energy switch. With respect to the dual-source dual-detector, as shown in FIG. 1, such a system consists of two sets of X-ray sources and detectors, i.e., the system comprises a high energy X-ray source 11, a high energy detector 12, a low energy X-ray source 21 and a low energy detector 22, and the high energy X-ray source 11 and high energy detector 12 intersect with the low energy X-ray source 21 and a low energy detector 22 at a angle of 90°. During data acquisition, these two sets of X-ray machines emit radiations of different energies (KVp), and the corresponding detectors collect data independently, so two groups of projection data, i.e. high energy projection data and low energy projection data, are obtained. However, the dual-source dual-detector system is very expensive, besides, it has high design requirements for the stability and strength of the mechanical structure of the rotating rack. In addition, with respect to the single source double layer detector, as shown in FIG. 2, in the design of such a system, a low energy filter and a detector (high energy detector in FIG. 2) are added behind the detector (low energy detector in FIG. 2) of the conventional single energy CT, thereby forming a dual-energy detector. When X-rays penetrate the first detector (low energy detector) and the filter, the low energy portion of X-rays is filtered out, and the high energy portion of X-rays reaches the second layer detector (high energy detector). Both detectors work simultaneously so as to collect two groups of projection data, i.e. low energy projection data and high energy projection data. However, the cost for such single source double layer detector is also high.
Moreover, with respect to the way of fast energy switch, such a system needs to use a special X-ray machine which can enable fast switch of high voltage and alternate emission of radiations of different energies (KVp). FIG. 3 is a schematic drawing of a dual-energy CT for realizing fast energy switch in a dual-energy CT system. As shown in FIG. 3(B), acquisition of high and low energy data can be realized by quickly switching the high voltage value of the X-ray machine. In this system, the rack rotates normally during scanning at a rotation speed of, for example, 0.5 s/rotation, with 1000 times of sampling being performed in each rotation, then the high voltage of the X-ray machine will be switched once in each sampling, and the detector will read data twice, high energy projection data in the first time and low energy projection data in the second time. At this time, the X-ray machine high voltage is switched 1000 times in one rotation, i.e. 0.5 second. However, in the fast energy switch system, a new type X-ray machine needs to be used, so said way of implementation has a high cost and is hard to be popularized and applied.
As mentioned above, the manufacturing costs for the above three types of dual-energy CTs are much higher than that of the conventional single energy CT, so they can hardly be popularized and used in common detections. In addition, the dual-energy CT cannot accurately reflect the true processes of the X-rays and the substance, so the result of reconstruction of some substance having characteristic absorption has poor accuracy, in contrast, the multi-energy CT is likely to solve this problem. Therefore, the multi-energy CT imaging system has gained extensive attention.